1. Field of the Invention
The present invention relates to a finder optical system used for an image pickup apparatus.
2. Description of the Related Art
A single lens reflex camera has a focusing glass arranged at a position (a primary imaging plane) conjugate to an imaging sensor which shoots an object image passing through an imaging lens. The light beam of an object image is collected in a direction toward an eyepiece by the action of the collection surface (Fresnel lens) of the focusing glass. The light beam is then acted upon by the diffusion surface (micro lenses) of the focusing glass to be diffusion light, and finally reaches the eyepiece.
An image of a part of the diffusion light is secondarily formed by a photometry lens on a photometry sensor which measures the brightness of an object. The use of this phenomenon makes it possible to measure the brightness of an object. As described above, a single lens reflex camera is designed to cause the light beam of an object image formed on its focusing glass to reach both of its finder optical system and its photometric system. This design requires improving photometric performance while maintaining a sufficient brightness of the finder by efficiently distributing the light beam of an object image which passes through the focusing glass in directions toward the eyepiece and the photometry lens.
Japanese Patent Laid-Open No. 2007-206169 discloses a configuration in which photometric properties are improved by the arrangement in which two kinds of Fresnel lenses whose focal lengths and light collection directions are different from each other are combined. Japanese Patent Laid-Open No. 2009-3423 discloses a configuration in which photometric properties are improved by the arrangement in which the center of a Fresnel lens is displaced toward a photometry sensor.
The collection surface (the Fresnel lens) of a focusing glass is a circle concentric to the center of the optical axis of an object image. The power of the Fresnel lens (Fresnel angle) is set such that the light beam of the object image is primarily collected to the eye point of the optical path of a finder designed for use in observation.
In the configuration disclosed in Japanese Patent Laid-Open No. 2007-206169, since two Fresnel lenses whose Fresnel angles are different from each other are arranged in combination, the light beam of an object image is collected to different eye points depending on each Fresnel angle. This results in uneven distribution of the quantities of light incident on an object observed by the finder. Likewise, in the configuration disclosed in Japanese Patent Laid-Open No. 2009-3423, since the center of a Fresnel lens is displaced toward a photometry sensor, the light beam of an object image is displaced toward the photometry sensor. This results in uneven distribution of light quantities especially at the periphery of the field of view of a finder. As described above, it is difficult to efficiently distribute a light beam to both a finder and a photometry sensor with the configurations of conventional technologies.